The present invention is drawn to an environmental barrier coating and, more particularly, an environmental barrier coating applied to a silicon containing substrate.
Silicon based ceramics exhibit accelerated oxidation rates in high temperature, aqueous environments such as for example, the combustor and turbine sections of gas turbine engines. In order to reduce the rate of oxidation on silicon based substrates used as ceramic components in such environments, significant effort has been given to providing environment barrier coating, i.e., barrier layer(s), for the silicon based substrates so as to increase the service life of such component parts.
With reference to FIGS. 1a and 1b, prior art environmental barrier coatings form a composite 10 comprising a silicon based substrate 12, a bond coat or layer 14 which comprises a dense continuous layer of silicon metal, a barrier layer 16 which comprises either an alkaline earth aluminosilicate based on barium and strontium, or a yttrium silicate and a top coat or layer 18 which comprises a refractory oxide and/or silicate or other metal oxide such as, for example, zirconium oxide. In addition, an intermediate layer 20 may be provided between the 14 bond coat and the barrier 16 and/or between the barrier layer 16 and top layer 18. The intermediate layer comprises, for example, a mixture of the barrier layer material with an additional oxide such as mullite. These prior art environmental barrier systems have proved to be protective with respect to oxidation of the silicon based substrate and, in addition, adherent. However, it has now been found that certain mechanical properties of the silicon substrate suffer as demonstrated by a significant reduction in 4-point bend strength at room temperature. It is believed that the loss of mechanical properties results from the cracking of the bond layer adjoining the silicon containing substrate which causes stress concentrations which initiate cracks in the substrate itself. The cracks in the bond coat or layer arise from many causes including differences in the coefficient of thermal expansion between the substrate and bond layer and stresses introduced as a result of rapid cyclic cooling and heating of the component parts.
Naturally, it would be highly desirable to provide an improved bond coat layer for silicon containing substrates which do not result in significant loss of mechanical properties.
Accordingly, it is a principle object of the present invention to provide an improved bond layer for an environmental barrier coating which overcomes the foregoing prior art problems.